Extensor expansion and extensor hood.

The extensor digitorum muscle inserts into the intermediate and distal phalanges of digits II-V via a tendinous aponeurosis (located on each individual digit) called the extensor expansion. By doing so, contraction of this muscle causes extension of the MCP and IP joints.

The expansion itself consists of two parts: a triangular, annular “hood” located around the MCP joints and proximal phalanges; and a single median band that runs dorsally along the intermediate phalanges to insert into the distal phalanges. This band is connected to the fibrous digital sheath on the anterior sides of each finger by retinacular ligaments.

The extensor exapansion is also the insertion of the lumbricals and interossei. The lumbricals insert onto the lateral hood (aka “extensor hood” or “dorsal hood”) of the expansion. Because of this, the lumbricals assist in extending the IP joints during MCP extension and are responsible for this extension during MCP flexion (which they assist in as well).

The interossei have a partial attachment into the expansion hood. Due to this, they assist the lumbricals in their actions (as well as ABduction/ADduction).

Pictures from http://home.comcast.net/~wnor/lesson5musofpostforearm.htm, Wesley Norman, PhD, DSc.